Surface position location system and method

ABSTRACT

An electrographic sensor unit and method for determining the position of a user selected position thereon. The electrographic sensor unit includes a layer of a conductive material having an electrical resistivity and a surface, at least three spaced apart contact points electrically interconnected with a layer of conductive material, a processor connected to the spaced apart contacts and disposed to selectively apply a signal to each of the contact points, and a probe assembly, that includes either a stylus of a flexible conductive layer spaced apart from the layer, coupled to the processor with the stylus disposed to be positioned by a user in vicinity of a user selected position on the surface of the layer, or that position being selected with a user&#39;s finger on the flexible layer and to receive signals from the layer when the contact points have signals selectively applied thereto. The user selected position is determined by the processor from signals received from the stylus, or flexible layer, each in relation to a similar excitation of different pairs of the contact points under control of the processor. The conductive layer may be either two or three dimensional and may be a closed three dimensional shape. There may also be multiple layers with the processor being able to discern on which of those layers the user selected position is located. Further, provision is made to correct the calculated coordinates of the selected position for variations in contact resistance of each of the contact points individually. Additionally, a nonconductive skin having selected graphics printed thereon, such as a map, can be placed over the layer and the proces-sor further convert the calculated coordinates of the selected position to coordinates that relate to the graphical information printed in the skin, and even electro-nically (e.g., audio or visual) present information to the user relative to the graphical location selected as the selected position.

This application is a Continuation-In-Part application of an earlierfiled co-pending patent application with the same title filed on Feb.15, 1996, and given Ser. No. 08/601,719 which includes as an inventorthe inventor of the present invention.

FIELD OF THE INVENTION

The present invention relates to a system and method for determining alocation selected by a user on a surface and providing information tothe user that has been determined to be relative to that location. Inparticular the present invention relates to position detection devicesthat are able to detect positions on a surface of two and threedimensional objects that have complex shapes. Additionally it relates toposition detection devices in which the object may be turned, rotated orotherwise manipulated relative to the rest of the position detectionsystem. Further, the present invention relates to provision of a groundpoint on the pointing device to ground the user to the system tominimize noise input to the system processor and potential error inposition identification.

BACKGROUND OF THE INVENTION

A variety of technologies exist to determine the position of a stylus,or even a finger, placed on a surface. One technology is a grid ofhorizontal and vertical wires that are placed below the surface of aflat tablet or over the surface of a display device and emit positionindicating signals which are detected by a stylus. Two devices usingthis type of technology are described in U.S. Pat. Nos. 5,149,919 and4,686,332 to Greenias, et al. Applications using these devices arecomputer input drawing (or digitizing) tablets, and touch-screen displaydevices.

In another technology, surface acoustic waves are measured at the edgesof a glass plate and are used to calculate the position on the platethat was selected by a finger or a stylus. Applications include high usetouch screen kiosk displays where a conductive overlay technology wouldwear out.

Yet other technologies include the use of light pens as opticaldetectors. Additionally a frame around a flat display with an array oflight emitters and detectors around the edge of the frame, may be usedto detect when a finger or stylus is near the display surface. Thesetechnologies are limited to displays or flat surfaces.

Position detectors such as the devices disclosed in the Greaniaspatents, that use many conductors arranged in a grid, are not wellsuited to a complex shaped surface of either two or three dimensions.There are, at a minimum, difficulties in positioning and shaping theconductors to fit the contours of a complex shape.

Another similar device is a grid of horizontal and vertical wires placedover or beneath the surface of a flat display device that usescapacitive coupling of a stylus or finger. In this device, thecapacitive coupling transfers position indicating signals from one wireto another which can be used to calculate the position of the coupling.Computer input tablets, as well as finger pointing mouse replacementtablets, use this technology.

In another technology, a rectangular homogenous transparent conductor isplaced over the surface of a display device and bar contacts on theedges of the transparent conductor charge the conductor. Capacitivecoupling of a stylus or a finger to the transparent conductor causes theconductor to discharge while sensors attached to the bar contactsmeasure the amount of current drawn through each of the contacts.Analysis of the ratios of the currents drawn from pairs of contacts onopposing sides of the rectangle provide an X-Y position on the panelthat was selected by the user. A device of this type is described inU.S. Pat. No. 4,853,498 to Meadows, et al. An application of this deviceis a touch-screen display.

A similar technology uses a rectangular piece of extremely uniformresistive material with a series of discrete resistors along the edgeand is mounted on a flat surface. A voltage differential is applied tothe row of resistors on opposing sides of the rectangle and in atime-division manner the voltage differential is applied to the row ofresistors of the other two opposing sides. The position indicatingsignals are either received by a stylus, or by a conductive overlaywhich can be depressed to contact the surface of the resistive material.One variety of this device is described in U.S. Pat. No. 3,798,370 toHurst.

The devices described in U.S. Pat. Nos. 4,853,498 (Meadows, et al.) and3,798,370 (Hurst) drive a homogenous rectangular resistive overlay withbar contacts or a string of resistors along each edge. These approachesrely upon the regular shape of a rectangle in order to work. The shapeand placement of the contacts provide the means to detect portions ofthe surface within a rectangular subsection of the resistive material ofthe surface. Other simple shapes may also be feasible with bar andresistor string contacts but in complex shapes they can create areasthat cannot be distinguished (e.g., shapes with concave edges such as acircle or ellipse can not be accommodated by either the Meadows or theHurst approaches). The use of bar contacts or strings of resistors alongsubstantially the entire edge of an object limits their usefulness onobjects where the position on the entire surface needs to be detected.The locations directly beneath each bar electrode and between each baror spot electrode and the edge of the object are not detectable in thesedevices.

The devices described in U.S. Pat. Nos. 4,853,499 (Meadows, et al.) and3,798,370 (Hurst) do not take into consideration the effects of contactresistance. The resistance between the contacts and the homogenousresistive material may be substantial relative to the resistance of thehomogenous material. Additionally the contact resistance may vary fromelectrode to electrode or change due to mechanical or environmentalstress. The Meadows and Hurst devices rely on contacts of known, orconstant resistance, which constrains the use of materials and contactapproaches. Any variation in contact resistance or changes in contactresistance due to environmental factors are not accounted for and resultin detection errors.

Further, Meadows loads the surface with a capacitively coupled stylusand determines position by measuring the current drawn from the drivingcircuits. The Meadows device requires four receiver circuits toaccomplish this.

The Meadows device is susceptible to the effects of unwanted phantomstyluses coupling to the surface. Phantom styluses such as rings orfingers may couple to the active surface instead of, or in addition to,the actual stylus. These phantom styluses cause detection errors becausethe changes that they also produce cause changes in the driving circuit.

In applications where the object containing the grid needs to berotated, or the electronics and the object are physically spaced-apartfrom each other, a large number of conductors must be coupled to thesystem, or between the elements of the systems, through connectionmechanisms that may allow rotation or other movements. Such cables forthe systems of the prior art would be rather large and cumbersome.Further, connectors with a large number of contacts are expensive andreduce the overall reliability of any system that requires them.Contacts that allow rotation, such as slip rings or commutators, becomeprohibitively complex and expensive as the number of connections risesabove a small number. Additionally, the multiple circuits required todrive grid arrays are complex and costly to manufacture. Acoustic wavedetectors provide a rugged position detection mechanism but are costlyto implement. Light wave detection mechanisms are limited to flatsurfaces and are susceptible to dust and insects blocking the lightpaths. It is believed, however, that the present invention solves theseproblems.

In today's modern environment there are many sources of electro-magneticenergy, both naturally occurring and man-made. Some examples of thesources of such energy in the earth's atmosphere are static electricity,electrical storms, heat lightning, radiation from outer space, andman-made radio waves. Each of these acts and interacts with each othercausing interference and background noise to each other, depending onthe intensity of the background or interfering signal. Thus, as is wellknown in devices that utilize an antenna as a device to detect an inputsignal, these atmospheric signals may interfere with the ability todetect and receive a signal of interest. It is also known that insystems with a hand-held antenna probe, the human body acts as a largerantenna with a signal from the person holding that probe added to thesignal of interest detected by the hand-held probe. That added signal,and the multiple frequencies that it includes is also known topotentially add a level of inaccuracy in such a system, if the desiredsignal can be detected at all. To overcome that unwanted interferencemany elaborate circuits have been devised to suppress those interferencesignals "picked-up" by the human user from impacting the performance ofthe system.

SUMMARY OF THE INVENTION

The present invention includes various apparatus and methods fordetermining a user selected position on an electrographic sensor unit.In the most general terms the electrographic sensor unit of the presentinvention includes a layer of a conductive material having an electricalresistivity with K spaced apart contact points electricallyinterconnected therewith, a processor connected to the K spaced apartcontacts and disposed to selectively apply a signal to N of the Kcontact points where N has an integer value of 3 to K, and a probeassembly, including a stylus or a flexible conductive layer placed overthe layer, coupled to the processor, the stylus disposed to bepositioned by a user in vicinity of the user selected position on thelayer, or the user to point a finger at the flexible conductive layer.In turn, the stylus, or the flexible conductive layer receives signalsfrom the layer when the contact points have signals selectively appliedthereto by the processor with the user selected position beingdeterminable by the processor from the signals received from the stylus,or flexible layer, each in relation to a similar excitation of (N-J)different pairs of the K contact points under control of the processor,where J is an integer of 2 to (N-1).

Additionally, where the electrographic sensor includes more than oneconductive layers that are each electrically isolated from each other,in the most general sense M conductive layers, the present invention isalso able to discern which of those layers contains the user selectedposition. Here, each layer has K spaced apart contact pointselectrically interconnected with the corresponding layer of conductivematerial where N of the K contact points on each layer are used tolocate the user selected position and where N has an integer value ofthree to K. The processor is similarly disposed to selectively apply asignal to each of the N contact points of each of the M layers and todetermine which of the M layers and position coordinates of the userselected position on the corresponding one of the M layers incooperation with a means for detecting and delivering a signal from theuser selected position on the selected layer of the electrographicsensor unit to the processor.

The identification of the selected layer is accomplished by sequentiallyapplying a first selected signal to all of the K contact points on eachof the M layers in turn and measuring a first measured signal at saiduser selected position for each of the M layers individually with thefirst measurement corresponding to each one of the M layers being thesignal received by the means for detecting and delivering when all ofthe contact points on that layer has the first selected signal appliedto that layer's contact points.

Next, a second measured signal is measured at the user selected positionon the user selected layer for each of the M layers with each of the Kcontact points on each of the M layers open circuited, followed by thesubtraction of the second measured signal from the first measured signalfor each of the M layers to form M difference values.

Those M difference values are then each compared against a preselectedthreshold value to determine which one of those M difference values isboth greater than that selected threshold and which exceeds it by thegreatest value. The layer associated with the difference value thatsatisfies those conditions is then identified as the layer that containsthe user selected position. Then once that determination is made thecoordinates of the user selected position on that layer can bedetermined as discussed above.

The present invention also includes techniques for compensating forcontact resistance in each of the contact points on the conductivelayer, as well as forming the conductive layer into a two or threedimensional shape which may be open or closed. Further, the presentinvention includes the placement of a conductive skin over the outersurface layer with that skin having a graphical representation thereonand the present invention having the capability to convert the positioncoordinates of the user selected position from the coordinates of theconductive layer to those of the graphical representation. Such agraphical representation may be that of a map or a globe, even amythical map or one of a star or another planet. Carrying this one stepfurther, those graphical coordinates may also be used to electronicallydeliver information that has been prestored in memory relative to theselected graphical coordinates to the user.

In actual application the present invention can take many forms from aconductive layer with or without a non-conductive layer thereon and astylus for use by the user to select a position on the layer, to amulti-layer structure with a conductive bottom layer, a non-conductivecompressible inner layer, and a flexible conductive top layer where theuser presses the top layer toward the bottom layer and the point atwhich the top and bottom layers are closest together is determined to bethe user selected position. Further, various designs are proposedwherein the actuation and measured signals are either AC of a selectedfrequency or DC.

The present invention also includes a probe assembly with a cable withtwo conductors. The proximate end conductor is coupled to the processorand the proximate end of the other conductor is connected to a signalneutral point. The stylus in turn is coupled to the cable andincorporates therein the distal ends of two conductors with the distalend of the conductor coupled to the processor disposed to receivesignals from the layer when the contact points have signals selectivelyapplied to them and the user positions the stylus in vicinity of aselected point on the surface. The distal end of the other conductor isdisposed to be contacted by the user when holding the stylus to connectthe user to the signal neutral point. To maximize the probability thatthe user holds the stylus making contact with the contact point, it islocated externally and positioned to be contacted by the user during useof the stylus. Further improve that probability, and to increase thecomfort of holding the stylus, an electrically conductive contact of aflexible conductive polymer is placed to encircle the stylus at aposition to maximize the user's comfort when holding the stylus.

Thus, to fully explain the scope of the present invention, a detaileddiscussion of various embodiments is offered in the Description of thePreferred Embodiments below. However it must be kept in mind that thatdiscussion is not an exhaustive discussion and variations on the manythemes that are presented are also considered to be part of the presentinvention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a simplified block diagram of a generalized embodiment of thesystem of the present invention.

FIG. 2 is an illustration of the position location algorithm of thepresent invention for a two dimensional surface shape.

FIG. 3 is similar to FIG. 2 however the illustration is for a threedimensional shape.

FIG. 4 is a block diagram of a first embodiment of the presentinvention.

FIG. 5 is a block diagram of a second embodiment of the presentinvention.

FIG. 6 is a block diagram of a third embodiment of the presentinvention.

FIG. 7 is a block diagram of a fourth embodiment of the presentinvention.

FIG. 8 illustrates the restrictions on the placement of contact pointsto be able to determine position with only three contacts.

FIG. 9 illustrates three contact points that can not be used todetermine position on the surface.

FIG. 10 is a partial embodiment wherein a multi-layer compressible touchsurface is disclosed in lieu of the use of a stylus as, for example, inFIG. 4.

FIG. 11 is a schematic representation of an embodiment of the presentinvention adapted to be an interactive globe that incorporates aspherical conductive surface.

FIG. 12 is a schematic representation of an embodiment of the presentinvention adapted to be an interactive globe that incorporates twohemispherical conductive surfaces.

FIG. 13 is a prior art embodiment of how a potential interfering signal,from the user holding the antenna stylus is suppressed.

FIG. 14a is a simplified diagram of the stylus and shielded cable of thepresent invention.

FIG. 14b is another embodiment of the stylus and shielded cable of thepresent invention that grounds the user to the system of the presentinvention.

FIG. 14c is still another of the stylus and shielded cable of thepresent invention that grounds the user to the system of the presentinvention.

FIG. 14d is a partial cut-away view of the stylus design of FIG. 14c toillustrate the internal positioning of the cable shield and theconductive grip of the stylus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a system and method for determining alocation on a two or three dimensional surface of any shape selected bya user, as well as providing access to data storage locations orinformation stored therein that is relative to that location. Morespecifically, the present invention determines the location informationin the form of coordinates on a predefined coordinate system. Thatlocation information then serves as an address to locations within thememory of an associated microprocessor subsystem. That location, oraddress may in-turn be used to retrieve previously stored datapertaining to the corresponding location on the surface, to store datapertaining to the corresponding location on the surface, to modify thebehavior of the system incorporating the present invention, or to bepresented to the user on a conventional display or printer device.

In simple shaped surfaces, such as a rectangle, a minimum of three smallelectrical contacts mounted on the edge of the surface are needed. Onmore complex shaped surfaces the minimum number of electrical contactsmay increase to enable the system to determine between multiplelocations on the surface as to which one that the user is indicating. Ineach configuration of the surface, the contacts need to be positionedsuch that all locations on the surface can be individually identified.

Through the use of small contacts and driver/receiver techniques, thepresent invention is able to compensate for differences in the contactresistance of each of the contacts. The differences that can becompensated for include differences between contacts on the samesurface, differences between the contacts on one surface versus those onanother surface using the same electronics, as well as changes in thecontact resistance of individual contacts over time due to mechanicaland environmental stresses.

The present invention determines a user selected position on the surfaceby measuring the unique position indicating signals with a receiver asdiscussed below. For either two or three dimensional objects, thepresent invention only requires a single receiver circuit.

In the various embodiments of the present invention, the stylus does notload, or negligibly loads, the transmitters and a signal level at thepoint on the surface that is touched by the stylus is measured ratherthan the changes in the driving circuit as in the Meadows device.Additionally, potentially phantom styluses such as fingers and rings,that have a dramatic effect on the operation of the prior art, only havea negligible loading effect on the transmitter of the present invention.Thus the present invention is immune to phantom styluses.

In the present invention the active surface can be made of a conductivepolymer composite (conductive plastic), or a conductive coating on anon-conductive material. This has substantial cost advantages over theprior art since no overlays or embedded wires are needed, and since thesurface itself provides the necessary structural support. Devicesincorporating the present invention would typically include a surface ofa conductive polymer composite molded or vacuum formed that does notrequire any additional structure thus resulting in an additional cost ofonly the carbon-polymer material, or the applied conductive coating.Furthermore, the formation of the sensitive surface by injection moldingallows for easy creation of touch sensitive complex shapes. The use of acarbon-polymer composite material as both an element in the positionlocation system and the structural support provides a rugged andreliable system. Carbon-polymer composite materials are inherentlyrugged and the system of the present invention employs a single layer ofsuch material, rather than a multi-layer system where the bondingbetween the layers may deteriorate and the layers separate.

A minimum of three contacts are needed to drive an entire surface of asimple object (e.g., a rectangle, circle or ellipsoid). Additionalcontacts may be used for complex objects or to provide increasedresolution for simpler shapes rather than increasing the sensitivity ofthe circuitry. The low number of contacts and therefore wire count,leads to low cost, ease of manufacturing, and enables remote or moveablesurface applications (e.g., a rotating globe).

An advantage to the use of a conductive polymer material for the surfaceis that it allows the contacts to be mounted to the back or inside ofthe surface, and to thereby achieve a 100% active front or outsidesurface.

Additionally, the present invention includes unique surface drivetechniques that can compensate for unknown and variable contactresistance. Various contact types and mechanical connection mechanismscreate contact resistances which vary substantially between contacts,and vary over time with mechanical and environmental stresses such asmovement, temperature and aging. Other technologies rely on contacts ofknown, or constant, contact resistance with any uncompensated change incontact resistance resulting in position detection errors.

The present invention permits the use of various mechanisms tocompensate for differences and variations in contact resistance. Each ofthose mechanisms may be used and each provides its own advantages. Onepossible mechanism involves using two electrodes as each contact, withthose electrodes being close together and electrically interconnectedbut not touching. The first of those electrodes in this configuration isattached to the signal drive source and the second of those electrodesprovides a high impedance feedback path. In this configuration thesignal drive source is adjusted so that the signal level at the secondelectrode is of a desired value thus providing a known signal level at aknown point on the surface independent of the contact resistance. Thedrive method here also provides automatic adjustment for changes in theresistive material over time and temperature, as well as variations incontact resistance.

A second possible mechanism has just one electrode per contact andmeasures the value of the resistance of each contact to the resistivematerial of the surface. In such a system having three contact points,A, B and C, a signal level measurement is made at point C through a highimpedance path while a signal of a known level is applied between pointA and point B. Similar measurements are then made at point B with thesignal applied between point C and point A, and at point A with thesignal applied between point B and point C. Thus, knowing the positionsof the contacts on the surface and the resistivity of the surfacematerial, the contact resistance between points A, B, and C and thesurface material may be calculated as discussed below with respect toFIG. 6.

Additionally, the present invention incorporates the use of amulti-state drive sequence to provide quick measurement and on-the-flycalibration for improved accuracy. The stylus is used to make severalsignal measurements at a point on the surface of the object selected bythe user. First a measurement is made with no signals applied to thecontacts to determine a baseline DC offset and ambient noise level forthe surface, for purposes of discussion here this is called DC-OFFSET. Asecond measurement is made with a signal applied to all of the contactsto determine the full-scale signal value, for purposes of discussionhere this is called FULL-SCALE. Another measurement is then made byapplying a signal to one pair of contacts to create a signal levelgradient across the surface between those two points, for purposes ofdiscussion here call this the X axis and the measured value X. A signalis then applied to another pair of contacts to create a signal levelgradient in another direction, for purposes of discussion here call thisthe Y axis and the measured value Y. The following calculations are thenmade by the system to determine the selected location along the sodefined X and Y axes on the surface.

    P.sub.x =(X-DC-OFFSET)/(FULL-SCALE-DC-OFFSET)              (1)

    P.sub.y =(Y-DC-OFFSET)/(FULL-SCALE-DC-OFFSET)              (2)

The actual position on the surface can then be determined from P_(x) andP_(y) by using a mathematical, or empirically determined, model of thesignal level gradients for the surface material.

In the present invention the basic items required (i.e., the algorithmand conductive material) have been around for quite some time. The basisfor the algorithm dates back centuries. Materials similar to what issuggested for the surface material here, having similar electricalproperties have also been around for decades.

The basis of the algorithm of the present invention is the use oftriangulation to determine the location of the point on the surface ofthe object. Triangulation is defined as

"The location of an unknown point, as in navigation, by the formation ofa triangle having the unknown point and two known points as thevertices." (The American Heritage Dictionary of the English Language,Third Edition)

Triangulation is a basic tenet of trigonometry and its use in findingthe location of a point on the surface of an object has been used forcenturies. It is used in applications such as celestial navigation,surveying, the global positioning system (GPS), and seismology.

In the present invention, as is the case in triangulation, position isdetermined by measuring the relationship at a point of interest to twoknown points. The relationship is determined from the received signallevel at the stylus while injecting signals of known levels at the firsttwo fixed points. All points on the surface that would have that signallevel create a line of possible positions. Another relationship isdetermined using another two fixed points (a different pair of contactshowever one contact can be one of those that was included in the firstpair of contacts) and another received signal level from the stylus. Theintersection of the two lines of possible positions from the twomeasurements thus tells us where the stylus touched the surface. Forsome surfaces this may be unique, such as a two dimensional surface or ahemisphere with the contacts mounted on the edge or at the equator.

In theory any position in three dimensional space can be uniquelyidentified by its distance from four non-coplanar known points, whilethe number of known points required may be reduced in some cases if thepossible positions in three dimensional space are constrained. For thepurposes of the present invention the position of interest isconstrained to lie on the surface of the known shape of the surface. Fora shape such as a rectangle or a circle, a position on the surface maybe defined by its distance from three known points on that surface,provided the known points are either all on the edge of the surfaceshape or not collinear. For the continuous surface shapes of spheres orellipsoids, a position on the surface of the shape can be defined by itsdistance from three known points, provided the plane defined by thethree known points does not include the center point of the shape. For acylindrical shape a position on the surface can be defined by itsdistance from three known points, provided the plane defined by thethree known points does not cross the center line of the cylinder.

For a relationship to be determined between a contact and a point on thesurface, the point must be in the field of view of a contact pair. Thatis, as shown in FIG. 8, for any point X to be in the field of view for apair of contacts A and B, the included angle, A_(i), between vectorsdrawn between A and B, and A and X, as well as the included angle,B_(i), formed by vectors drawn between B and A, and B and X, must bothbe less than 90°. Additionally the surface must contain electricallyconductive material between points A and X and between X and B. FIG. 9illustrates a situation where point X is not in the field of view ofpoints A and B since included angle B_(i) is greater than 90° eventhough included angle A_(i) is less than 90°.

In practice more contact points may be used due to the finite resolutionof real measurement devices. Another factor that may increase the numberof contacts is cost. A trade off may be made between the resolution ofthe receiver and transmitter circuits, and the number of contactsbetween which the signal is applied to the surface for the measurements.If more contacts are used that are closer together then the resolutionof the transmit/receive circuit may be reduced.

The use of resistivity in materials to measure distance or position hasbeen around for a number of years. An early example is the use ofrotating, or sliding, potentiometers to determine the position of a knobor a slide.

Conductive polymers that could be employed by the present invention havebeen around at least since 1974 when CMI, an early producer ofConductive Polymer Composites, was acquired by the 3M Company.

At a minimum the materials and algorithms utilized by the presentinvention have been readily available for 20 years, and in alllikelihood longer. However, the literature does not teach or suggest thecombination of those elements to produce a device like the presentinvention, in fact all of the known references teach away from thistechnique.

In FIG. 1 the basic components of the user selected position locatingsystem of the present invention are shown. They include a two or threedimensional conductive surface 10 (e.g., carbon loaded plastic or aconductive coating applied to a non-conductive surface) having aselected resistivity with three conductive contacts 12, 14 and 16affixed thereto. Each of contacts 12, 14 and 16 are connected viaconductors 24, 26 and 28, respectively, to processor 30. Also connectedto processor 30 is conductor 18 with a stylus 20 having a tip 22 affixedto the other end thereof for the user to use to indicate a position onsurface 10 that is of interest to that user.

Then, as in FIG. 2 when a user selects a point on surface 10 with stylus20, a series of measurements as described in general terms above aremade.

First, without any signals applied to contacts 12, 14 and 16, processor30 measures the DC-OFFSET value of the system with stylus 20;

Next an equal amplitude signal is applied to all three of contacts 12,14 and 16, and processor 30 measures the FULL-SCALE signal value withstylus 20;

The third measurement is made by applying a signal of the amplitude usedin the full-scale measurement to one of the three contacts, say contact12 with a second contact grounded, say contact 14, and the signalmeasurement made with stylus 20 which will be somewhere along anequipotential line between those two contacts (i.e., line X in FIG. 2);

A fourth measurement is made by applying the signal to, and grounding, adifferent pair of contacts, say 12 and 16, and the signal measurementmade with stylus 20 which will be somewhere along an equipotential linebetween those two contacts (i.e., line Y in FIG. 2), with the positionof stylus 20 being the intersection of lines X and Y; and

The values of P_(X) and P_(Y) are then calculated as in equations 1 and2 above.

In actual operation, each of those steps can be automated by processor30 without requiring the user to initiate specific measurements or toswitch signals.

The values of P_(X) and P_(Y) can then be used as an address to a memorywithin processor 30 from which information relative to the positionindicated with the stylus may be obtained. This same technique can alsobe used to determine the address in memory where data is to initially bestored for later retrieval, or as an address on a remote display that isto be activated for whatever purpose.

Each unique position on the surface is defined by a unique combinationof values of P_(X) and P_(Y). From the series of measurements describedabove, the position of the stylus on the surface may be expressed interms of P_(X) and P_(Y) which will be called the equipotentialcoordinates. Additional calculations may also be made to convert theposition from the equipotential coordinates to another coordinatesystem, if desired. The conversion requires a known mapping of theequipotential coordinates to the desired coordinate system. The mappingmay be determined mathematically for an object made from a homogenousconductive material or one where the resistivity distribution is known.For objects in which the resistivity distribution is not known, themapping of equipotential coordinates to the desired coordinates may bedetermined empirically. In either case, the mapping may be stored in themicroprocessor's memory and the conversion calculations performed by themicroprocessor.

FIG. 3 illustrates the same approach for determining the values of P_(X)and P_(Y) on the surface having a defining equation that is continuousover the entire surface, for example a hemisphere as shown.

Surface 10 of the present invention uses materials such as carbon loadedpolymers or conductive coatings (e.g., 3M Velostat 1840 or 1801) thatcan be easily molded into, or applied to, two or three dimensionalsurfaces, including surfaces having complex shapes. A minimal number ofdrive circuits and connections between that surface and the detectionelectronics further will reduce the complexity in both the electronicsand the mechanical aspects of coupling the surface to the electronics.

More specifically, several embodiments of the present invention aredescribed in the following paragraphs and illustrated starting with FIG.4.

The embodiment, shown in FIG. 4, includes a rectangular piece ofconductive material as sheet 100 (e.g., 12 inches×12 inches×0.125 inchessheet of a carbon loaded polymer such as 3M Velostat 1801). Theconductive material may also be composed of a non-conductive materialwith a conductive coating such as Model 599Y1249 from Spraylat Corp.

Affixed near the edge of sheet 100, and making electrical contactthereto, are contacts 102, 104, and 106. Connected between contacts 102,104 and 106 on sheet 100 and contacts 126, 128 and 130 of signalgenerator 122, respectively, are electrically conductive leads 108, 110,and 112.

Signal generator 122 includes a 60 KHz AC signal generator 124 thatfeeds amplifier 134 with the non-inverting output terminal of amplifier134 connected to three separate terminals (one corresponding to each ofcontacts 102, 104 and 106) of switch 132, and the inverting outputterminal of amplifier 134 connected to three terminals (onecorresponding to each of contacts 102, 104 and 106) of switch 136. Theneach of contacts 126, 128 and 130 are each connected to differentterminals of each of switches 132 and 136. In FIG. 4 each of switches132 and 136 are shown in the open position (i.e., no signal applied toany of contacts 126, 128 and 130).

In turn, the position of each of switches 132 and 136 is controlled viacables 138 and 140, respectively, from microprocessor 142 to permitmicroprocessor 142 to select which of contacts 102, 104 and 106 receivea 60 KHz signal through switch 132 via the associated control lead andwhich of contacts 102, 104 and 106 receive an inverted 60 KHz signalthrough switch 136 via the associated control lead.

When a 60 KHz AC signal is connected to one or more of contacts 102, 104and 106 that signal radiates through the conductive material of sheet100 and stylus 116 acts as an antenna when brought within proximity ofsurface 100. A signal detected by stylus 116 is in turn conducted to thesignal measurement stage 120 via shielded cable 118. In this embodimentstylus 116 is completely passive and could be fabricated as simply asconsisting of a plastic shell enclosing the end of shielded cable 118with the final 1/8 inches of cable 118, at the distal end of stylus 116,having the shielding removed to allow the center conductor of cable 118to be exposed to receive radiated signals. Thus, when the tip of thestylus is near the surface of conductive material 100, the radiatedsignal is received by the stylus antenna and provided as an input signalto signal measurement stage 120.

Signal measurement stage 120 includes a demodulator 144 that isconnected to cable 118 where the signal received by stylus 116 isdemodulated and the demodulated signal is then in turn presented as asignal level to an analog-to-digital converter (ADC) 146. ADC 146 thendigitizes that signal level and presents it to microprocessor 142.

The use of an AC signal in this embodiment makes it possible for stylus116 to receive signals radiated from the conductive material of sheet100 without being in direct contact with the conductive material ofsheet 100. This allows the conductive material of sheet 100 to becovered with a layer of a non-conductive material for protection frominevitable striking of the surface of sheet 100 with stylus 116, or forplacement of application specific graphics over the touch surface, andstill allow stylus 116 to act as an antenna to receive a signal fromsheet 100 at a selected point that is to be measured by the signalmeasurement stage 120.

Microprocessor 142 is encoded to direct the performance of a series ofmeasurements with different sets of contacts 102, 104 and 106 connectedto receive the 60 KHz signal, or the inverted 60 KHz signal.

Once a user has selected a position on sheet 100 of interest, the systemof the present invention performs a series of measurements in rapidsuccession (e.g., by time-division multiplexing) to determine thelocation to which stylus 116 is pointed and to provide the user with theinformation that is sought.

The first measurement, as outlined above, is here calledSignal_(OFFSET), and involves setting switches 132 and 136 to the allopen positions. Microprocessor 142 then reads the signal level fromsignal measurement stage 120 and assigns that value to Signal_(OFFSET)and saves that value in RAM 144.

The second measurement, as outlined above, is here called Signal_(FULL),involves connecting a 60 KHz AC signal to all of contacts 102, 104 and106 at the same time by the closure of all three sets of contacts inswitch 132. Microprocessor 142 then reads the signal level from signalmeasurement stage 120 and assigns that value to Signal_(FULL) and savesthat value in RAM 144.

Next, microprocessor 142 selects a pair of contacts, say 102 and 104,for use in the next measurement. Contact 102, for this discussion ispoint A and is connected to receive the 60 KHz AC signal via switch 132.The other of those two contacts, contact 104, which for this discussionis point B is connected to receive the inverted 60 KHz AC signal viaswitch 136. The third contact 106 is merely connected to open switchsections in both of switches 132 and 136. Microprocessor 142 then storesthe signal level from signal measurement stage 120 in RAM 144 andassigns that value the name Signal_(RAW-AB).

Between the energized contacts 102 and 104, a signal level equipotentialmap 114A could be drawn due to the effect of the distributed resistancein the conductive material of sheet 100. Signal equipotential maps suchas 114A, 114B, and 114C, including the shape and values of theequipotential signal level lines, are stored in ROM 146. As discussed inElectromagnetics, by John D. Kraus and Keith R. Carver, McGraw-Hill,1973, pp 266-278, these signal equipotential maps are created by findingthe unique solution to Laplaces equation (∇² V=0) that satisfies theboundary conditions of sheet 100 and each pair of contacts. There aremany methods of finding the solution to Laplace's equation for anobject, including, but not limited to, direct mathematical solutions,graphical point-by-point computer modelling, and empiricaldetermination. For homogenous conductive material and simple shapes, adirect mathematical solution may easily be obtained. For materials whosehomogeneity, shape or contact placement do not lend themselves to othermethods, empirical determination may be used.

In the empirical determination method, a coordinate system is chosen andoverlaid on the device. To determine the map for a specific pair ofcontacts, such as 102 and 104, the contacts are energized in the samemanner as for measuring Signal_(RAW) AB above. At each cross point onthe chosen coordinate system the value of Signal_(RAW) AB is measured.If the chosen cross point granularity is sufficiently fine theequipotential map may be extracted directly by finding the points thatcontain the same measured value. Otherwise the equipotential lines maybe calculated by interpolating between measured points.

For the third measurement, microprocessor 142 selects another pair ofcontacts, such as 102 and 106. Contact 102, as discussed above willagain be referred to as point A, is connected to receive the 60 KHz ACsignal via switch 132 and is the only one of the contacts so connected.The other contact 106, which for this discussion is referred to a pointC, is connected to the inverse 60 KHz signal via switch 136.Microprocessor 142 then records the signal level from signal measurementstage 120 and assigns that value the name Signal_(RAW-AC).

The two signals, Signal_(RAW-AB) and Signal_(RAW-AC), are affected notonly by the material resistance between the contacts but by a number ofother factors including the altitude of stylus 116 from the surface ofthe conductive material of sheet 100, the attitude or angle of stylus116, and changes in the circuitry from environmental changes, aging, orother factors. The signal, signal_(FULL), is similarly affected byaltitude, attitude, and circuitry changes but has a constant signalequipotential map, thus the value of Signal_(FULL) may be used tonormalize the values of Signal_(RAW-AB) and Signal_(RAW-AC) to removethe effects of altitude, attitude, and circuitry changes using thefollowing formula.

    Signal.sub.NORM =Signal.sub.RAW /Signal.sub.FULL           (3)

Both Signal_(RAW) and Signal_(FULL) are affected by certain changes inthe circuitry that produce a DC offset in the final values. Equation 3,if desired, may be modified to remove those effects as shown in equation4 below.

    Signal.sub.NORM =(Signal.sub.RAW -Signal.sub.OFFSET)/(Signal.sub.FULL -Signal.sub.OFFSET)                                       (4)

Applying either formula of equations 3 and 4 to each of Signal_(RAW-AB)and Signal_(RAW-AC), the normalized signals, Signal_(NORM-AB) andSignal_(NORM-AC), can be derived.

For example, using the predetermined signal map 114A and the valueSignal_(NORM-AB), the position of stylus 116 may be resolved to a singlesignal level line, such as 115, between contacts 102 and 104.

Using the predetermined signal map 114B and the value Signal_(NORM-AC),another signal level line in the signal map 114B can be determinedbetween contacts 102 and 106. The position of stylus 116 is thenresolved to the point, P, where the signal level line selected bySignal_(NORM-AB) in 114A crosses the signal level line selected bySignal_(NORM-AC) in 114B.

The use of the resolved point, P, is qualified by microprocessor 142 bycomparing the value of Signal_(FULL) to a predetermined threshold levelto determine if the received signal is valid. This threshold isgenerally determined empirically to satisfy the resolution requirementsof the application or the user. As the altitude of stylus 116 from thesurface of the conductive material of sheet 100 is reduced, the receivedsignal is stronger and the resolution of the position is more precise.Some applications such as drawing tablets, may want a specific altitudethreshold in order to match user expectations of operation. In theseapplications, users do not expect the system to acknowledge the stylusposition until the tip is in contact with the surface. Otherapplications may desire a higher or lower degree of resolution. Theapplication may select the altitude threshold that best matches it'srequirements. When a Signal_(FULL) threshold for a particularapplication is satisfied the resolved point, P, is considered valid.

The measurements outlined above are made in succession and eachmeasurement can typically be made within 4 msec so the entire sequenceis completed in 12-16 msec. This is important since the measurementsequence needs to be completed quickly so that any stylus positionchanges between the measurements are minimized. Substantially fastersample times may be used provided that the capabilities of the signalmeasurement device are selected accordingly.

To support an application that requires a series of stylus locations inquick succession to be measured, a sample time that is substantiallyfaster than the movement of the stylus needs to be chosen. Anapplication that would require successive stylus location detectionwould be an electronic drawing pad where the succession of points wouldform a line. An application of this type may require sample times on theorder of 200 microseconds.

In the embodiment discussed above, signal generator 122 produces a 60KHz AC signal, however, a DC voltage level could alternatively be used.With a DC signal level in lieu of the 60 KHz signal the ability todetect the position of the stylus without making contact between stylus116 and the conductive material of sheet 100 is eliminated. Since directcontact is made between the stylus and the material, the effects of thealtitude and attitude of the stylus no longer contribute to themeasurement of Signal_(RAW) since stylus altitude and attitude are thedominant source of variation in the measurement of Signal_(RAW). Theelimination of stylus altitude and attitude from the measurementreduces, or eliminates, the need to normalize Signal_(RAW) withSignal_(FULL).

More measurements (contacts 104 to 106, i.e., B to C) may also be madeto refine/confirm the point to which stylus 116 is being pointed with aminimum number of measurements. Microprocessor 142 could also beprogrammed to filter measurements to dampen changes made by movement ofstylus 116 and to increase resolution.

Synchronous detection techniques in the receive demodulatorsubstantially improve noise immunity. The received signal is multipliedby the transmitted signal with a FET switch (e.g., DG441). The resultingmultiplied signal is then integrated to determine the DC component. Itis the integrated signal that is presented to the ADC for conversion.The net effect of the multiplication and integration is that onlyreceived signals of the same frequency and phase to the transmittedsignal are seen. Such signals are considered to be synchronous with thetransmitter, and therefore the name synchronous demodulation. Effectivenoise immunity is accomplished since, in general, sources of noise willnot be synchronized to the transmitter, and therefore will not be seenafter multiplying and integrating. Only the desired portion of thetransmitted signal that has been detected by the receiving stylus willbe measured.

Special techniques can be used to enhance the accuracy near the edges ofa conductive surface. On surfaces of certain shapes, the lines ofequipotential may be nearly parallel near the edges, which tends toreduce the positional accuracy. Distance to the edge can be estimatedfrom Signal_(FULL) alone, since Signal_(FULL) tends to fall off somewhatnear the edge. Applying an estimate of the distance from the edge topoint determined by the intersection of two equipotential lines near theedge can help improve positional accuracy in some cases.

In cases where two electrically isolated surfaces terminate along thesame edge, such as the equator on a globe made of isolated Northern andSouthern hemispheres, similar techniques can be used to improvepositional accuracy near the edge. In such cases distance from the edgecan be estimated by comparing Signal_(FULL) from both surfaces, andusing the ratio of Signal_(FULL-A) to Signal_(FULL-B) to help eliminatethe effects of altitude and attitude.

Once the position indicated by the user is determined, the system mightbe employed in an application where information relative to thatposition has been prestored, or is to be stored, in the overall system.To enable that application, RAM 144, ROM 146, audio/video card 150 andCD ROM drive 156 are shown interfacing with microprocessor 142 via adata bus. For example, if surface 100 has an overlay of a map there maybe information prestored in ROM 146 or on a CD in CD ROM drive 156 thatcan be delivered to the user in either audio or visual form viaaudio/video card 150 and speaker 154 or monitor 152.

The contact resistance of the connections between contacts 102, 104 and106 and the conductive material of sheet 100 may play a significant rolein defining the absolute signal levels in the signal maps (114A, 114Band 114C). That contact resistance affects the absolute value of thesignal level but has only a minor effect on the shape or distribution ofsignal lines. In some cases the contact resistance between one contactand the conductive material of sheet 100 may be of a similar, or ahigher, value than the resistance through the conductive materialbetween different contacts. The resistance between a single contact andthe conductive material is also subject to change over time due tochemical, or mechanical factors. Contact to conductive materialresistance may also differ from unit to unit in a manufactured product.

To automatically compensate for contact-to-conductive materialresistance differences, which is addressed in the embodiment of FIG. 4by calculation, another embodiment of the present invention is shown inFIG. 5. As can be seen by the comparison of FIGS. 4 and 5 many of theelements of the two circuit embodiments are the same and connectedtogether in the same way, in particular, sheet 100, the signalmeasurement stage 120, microprocessor 142 and associated components,signal generator 124, amplifier 134, and switches 132 and 136. Theadditional elements in FIG. 5, which are described below, have beenadded to provide the automatic compensation for resistance differencesmentioned above.

The first difference between the two figures is in the structure of thecontacts affixed to sheet 100. In FIG. 5, stated in simple terms, asingle contact as shown in FIG. 4 is replaced with a connected pair ofcontacts. A first contact of each connected pair is used as the point towhich connection of the signal generator is made, while the secondcontact of the connected pair is used as the point at which measurementsof the signal level is made and at which adjustments of the signal levelbeing injected at the first contact in that connected pair is made sothat the signal level at the measured point is at a known level.

For example, contact 102 in FIG. 4 is replaced with connected pair 202aand 202b in FIG. 5. In this embodiment contact 202a could be a 0.0625inches diameter contact positioned at the same point on sheet 100 ascontact 102 in FIG. 4, and is used as the injection point of a signalinto the conductive material of sheet 100. Similarly, contact 202b couldbe a 0.0625 inches diameter contact positioned 0.25 inches from contact202a and used as the point at which the signal level is measured at theassociated point on sheet 100.

The second difference from the embodiment of FIG. 4 is the connection ofthe output terminal of each of two input terminal amplifiers 220, 224and 228 (e.g., MC4558) to contacts 202a, 204a and 206a, respectively.Each of amplifiers 220, 224 and 228 has the positive input terminalconnected to a different one of the output terminals of switches 132 and136. Each of amplifiers 220, 224 and 228 has the negative input terminalconnected to a different one of the "b" contacts of each connected pairattached to sheet 100 (i.e., contacts 202b, 204b and 206b).

When the input signal passes through the resistance of the contact, thesignal level decreases. If the resistance of the contact changes, thesignal level changes inversely proportionally to the change inresistance of the contact. Therefore if such a change in the inputsignal level is inversely compensated for in another way, any change ofsignal level resulting from a change in the resistance of a contact isnegated. Persons skilled in the art of closed loop feedback theory willrecognize that the "b" contacts of sheet 100 provide feedback to the "a"contact drive amplifier 202A, 204a and 206a, such that those amplifierscan sense any decrease in signal level due to contact resistance, andprovide the necessary signal boost to compensate for loss.

An alternate mechanism for compensating for contact resistance is todetermine the current value of the contact resistance and adjust theabsolute values in the signal map based on any change in contactresistance value. The embodiment shown in FIG. 6 performs that function.

Again comparing the embodiments of FIGS. 4 and 6, several similaritiescan be noted which include sheet 100 with contacts 102, 104 and 106,stylus 116 and shielded cable 118, signal measurement stage 120,microprocessor 142 and associated components, and signal generator 122.The new component here is four position switch 301 which providesselectability as to which signal is input to the input terminal ofdemodulator 144 of the signal measurement stage 120 under control ofmicroprocessor 142 via line 302. The four potential signal input sourcesare stylus 116 and any one of contacts 102, 104 and 106 on sheet 100.

For any position in the signal map between two points, any change in theresistance of any contact through which current is flowing will modifythe signal value observed. For example, for a predetermined, orcalculated, signal map such as 114A between contacts 102 and 104 in FIG.4, a change in the contact resistance at contact 102 will change theabsolute values in the signal map but not the distribution or shape ofthe signal map. If the contact resistance at 104 were to change and thenew contact resistance measured, the microprocessor could adjust thepredetermined, or calculated, signal map to compensate for the changedcontact resistance.

To measure and calculate the contact resistance changes at the threecontacts 102, 104, and 106 in FIG. 6, three additional measurements aremade. These measurements may be added to the sequence of measurements ofSignal_(FULL), Signal_(OFFSET), Signal_(RAW-AB) and Signal_(RAW-AC). Forthis discussion the contacts will be designated A, B, and C for contacts102, 104, and 106. For the first additional measurement themicroprocessor selects contact 102 to be connected to the 60 KHz ACsignal via switch 132, and contact 104 to be connected to the inverted60 KHz AC signal via switch 136. The signal measurement device isconnected to a fixed location, contact 106, via switch 301. Themicroprocessor then stores the signal level from the signal measurementstage in RAM as Signal_(C).

The second additional measurement is made with contact 102 connected tothe 60 KHz AC signal and contact 106 connected to the inverted 60 KHz ACsignal. The fixed point, contact 104, is connected to the signalmeasurement device. The microprocessor then stores the signal level fromthe signal measurement stage in RAM as Signal_(B). The third measurementis made with contact 104 connected to the 60 KHz AC signal and contact106 connected to the inverted 60 KHz AC signal terminal of amplifier134. The fixed point, contact 102, is connected to the signalmeasurement device. The microprocessor then stores the signal level fromthe signal measurement stage in RAM as Signal_(A).

Thus, the measured signal levels can be defined by equations 5a-5c:

    Signal.sub.C =Signal.sub.IN  (X·R.sub.AB +R.sub.A)/(R.sub.A +R.sub.AB +R.sub.B)!                                      (5a)

    Signal.sub.B =Signal.sub.IN  (Y·R.sub.AC +R.sub.A)/(R.sub.A +R.sub.AC +R.sub.C)!                                      (5b)

    Signal.sub.A =Signal.sub.IN  (Z·R.sub.BC +R.sub.B)/(R.sub.B +R.sub.BC +R.sub.C)!                                      (5c)

where:

Signal_(IN) is the signal level injected between two contacts;

R_(AB), R_(AC) and R_(BC) are the bulk resistances of the materialbetween contacts A and B, A and C, and B and C, respectively;

X, Y, and Z define the distribution of the bulk resistance as seen atthe measurement point, between the two drive contacts; and

R_(A), R_(B), and R_(C) are the contact resistances at contacts A, B,and C, respectively.

The values of Signal_(IN), X, Y, Z, R_(AB), R_(AC), and R_(BC) areconstant values that may be measured and/or calculated for a particulardevice and stored in the microprocessors memory. That leaves a series ofthree simultaneous equations with three variables, i.e., R_(A), R_(B),and R_(C). The microprocessor then can solve those simultaneousequations for the values of R_(A), R_(B), and R_(C), and then themicroprocessor can adjust the signal value tables based on the newvalues of R_(A), R_(B), and R_(C).

An alternate mechanism to driving a pair of contacts and sensing with areceiver connected to the stylus is to use the stylus and one of thecontacts as the driving mechanism and to perform sensing with one of theother contacts. A sequence of measurements could be made where anothercontact is selected as the drive contact and yet another contact isselected as the sense contact.

An alternate drive and measurement method is provided by the use offrequency division multiplexing. Previously discussed methods include aseries of measurement steps separated in time. In a frequency divisionmultiplexing method, pairs of contact points are driven simultaneouslywith different frequency signals. Therefore the signal received by thestylus is a composite of those different frequency signals and thus isdistributed to multiple independent signal measurement devices (i.e.,sorted by frequency) that each measure the corresponding signalsimultaneously. The multiple measurement devices in this embodiment aredesigned to measure signals within narrow frequency bands. Thismeasurement method offers the possibility of measuring the position inless time however with a more complicated signal drive and measurementdetection system.

Several design tradeoffs may be made in the implementation of thepresent invention for use in a specific device. To enhance resolution ahigher resolution signal generation and measurement scheme may be used.Alternately the number of contact points may be increased and anenhanced algorithm implemented that uses subsets of the contact pointsto resolve stylus touches on different areas of the surface. Anotheralternative might be the selection of a conductive material andmanufacturing method that provides a more homogenous resistivity in thesurface. This increases the resolution and allows for calculated, ratherthan measured signal maps. If the material used is not homogenous,another way that higher resolution may be accomplished is by measuring amore comprehensive signal map that is stored in the microprocessormemory.

The embodiments described in FIGS. 4, 5, 6, and 7 include a stylus thatis tethered to the rest of the detection system by conductor 118. Thisconductor may be replaced with a communications link that does notrequire tethering the stylus to the system with a conductor. A low powerRF transmitter could be embedded or attached to the stylus and acompatible RF receiver attached to the signal measurement means. The RFtransmitter and receiver would then implement the communications linkthat conductor 118 provided.

The present invention may be extended to include other two and threedimensional shapes, both with a surface shape that smoothly changesslope (e.g., a sphere or a saddle shape) and shapes with sharp edges(e.g., a cube or a pyramid) so long as the resistive surface iscontinuous through those changes of slope and around those sharp edges.

In another embodiment as shown in FIG. 7, the position of stylus 116 ona sphere may be detected. In this embodiment a sphere 400, molded from aconductive material of the same type discussed for each of the otherembodiments, has four contacts 401, 402, 403 and 404 attached to it. Inorder to be able to individually distinguish each point on the surfaceof a closed three dimensional shape (e.g., a sphere) the contacts mustbe positioned such that each plane defined by each possible combinationof any three of those contact points does not pass through the center ofthe sphere. How close these imaginary planes can come to the center ofthe sphere (i.e., the placement of the contacts) is determined by theresolution of the signal measurement device and the precision of thepredetermined, or calculated, signal equipotential map that determinesthe point to which the stylus is pointed.

The calculation of position is therefore substantially the same asdiscussed with respect to a pair of contacts thus that discussion andthe claims also include this variation.

To resolve the position of stylus 116 on the two dimensional area of therectangular sheet 100 in the embodiment of FIG. 4, three measurements,Signal_(FULL), Signal_(RAW-AB), and Signal_(RAW-AC) were required since,as described above with respect to FIG. 2, the equipotential lines foreach of the AB and AC measurements can only cross in one point. For asphere as in FIG. 7, however, four measurements are required to fullyresolve the position. For example, if contact 401 is point A, contact402 is point B, contact 403 is point C and contact 404 is point D, ameasurement of Signal_(FULL) with all four points driven simultaneouslyis one measurement, and three measurements from the six possible paircombinations of the four contacts must be made, namely three of thepossible measurements Signal_(RAW-AB), Signal_(RAW-AC), Signal_(RAW-AD),Signal_(RAW-BC), Signal_(RAW-BD), or Signal_(RAW-CD). Calculating thethree Signal_(NORM) values as in equation (3) above and plotting thosevalues on the applicable signal maps will uniquely resolve all points onthe sphere. When two Signal_(NORM) values are plotted, the equipotentiallines intersect in two places on opposite sides of the sphere. The thirdSignal_(NORM) value is used to determine which of the two intersectpoints is the one to which the stylus is being pointed. Specifically, ifthe signal measured at the fourth point where used with the signal fromone of the other two points that were used to locate the first twoalternative points, that combination would also result in two possiblepoints on the sphere, however, one of those two points would correspondwith one of the two previously determined points and it is thatcorresponding point that is the actual point of interest on the sphere.

An alternative to using a stylus as the pointing device is the use of afinger as the pointing device. To enable this, a multi-layer materialconstructed with the inner layer being similar to the conductivematerial discussed in the previous embodiments may be used. Such asurface is illustrated in FIG. 10 with conductive layer 100 on thebottom, a flexible conductive layer 501 on top (e.g., a metal foil or athin layer of a conductive polymer), and a compressible non-conductivelayer 502 (e.g., silicon rubber or plastic foam) in-between layers 100and 501. Outer layer 501 may be metal, or some conductive material.

In this configuration, outer conductive layer 501 replaces the attachedstylus 116 as in FIG. 4 with outer layer 501 connected to the signalmeasurement device by conductor 118 (e.g., see FIG. 4). Thus, when auser touches outer layer 501, the middle non-conductive layer 502compresses and conductive outer layer 501 is brought closer toconductive inner layer 502. In that situation, the signal level receivedby outer layer 501 from the radiated signals on inner layer 100increases in much the same way as the signal level received by stylus116 increases as the altitude of stylus 116 is decreased relative tosurface 100 in FIG. 4. In the embodiment that utilizes the multi-layersurface, the position of the user's finger would be calculated in thesame way as the location of the stylus with a threshold value chosen forSignal_(FULL) in the signal valid determination step that corresponds toa fully depressed outer layer.

As mentioned briefly above with respect to FIG. 4, one application ofthe present invention might be an interactive globe of the earth, themoon, one of the planets, one of the stars, or even an artificial bodyor planet for an interactive game. Two potential implementations of sucha globe are illustrated in FIGS. 11 and 12. The primary differencesbetween the embodiments of those figures is that in FIG. 11 theconductive surface is a sphere, and in FIG. 12 the conductive surface isimplemented with two hemispheres.

FIG. 11 illustrates the system disclosed above with respect to FIG. 7being modified to be a world globe. Thus, the electronics in the lowerportion of FIG. 11 have the same reference numbers as, and operate inthe same way described, in FIG. 7. In FIG. 11 there is a conductivesphere 603 with four contact points 604, 605, 606 and 607 on the insideof sphere 603, with each of the contact points connected, respectively,to one of the four insulated conductors of cable 608 at one end of thoseconductors. Cable 608 exits sphere 603 through a small hole in thebottom of sphere 603 with the other end of the conductors of cable 608interconnecting with the corresponding sections of switches 422 and 432.

To provide the geographic details of the globe, two vinyl skins 601 and602, shown here as representing the northern and southern hemispheres ofthe earth, are placed over sphere 603. Thus when a user uses stylus 116to point to a location on the globe, the electronics determines thecoordinates of that selected location as described above in thediscussion with respect to FIG. 7 since the electronics here are asdescribed there. The unique location on the surface of the globe is thusdefined by the equipotential coordinates which can then be mapped bymicroprocessor 142 (e.g., by means of a look-up table) into globalcoordinates (e.g., longitude and latitude) that correspond to theselected position on the globe.

A database containing features of interest in the world, such as countrylocations and names, capitals, and populations can be prestored in RAM144 relative to what ever coordinate system is desired. Thus, when auser selects a point on the globe with stylus 116, microprocessor 142determines the coordinates of that position and causes the retrieval ofinformation relative to that position from the database to be presentedto the user via, for example, audio/video card 150 and speaker 154.

An alternative implementation of a world globe is illustrated in FIG. 12where conductive hemispheres 701 and 702, that are electrically isolatedfrom each other, provide the conductive surfaces for the globe. Herehemispheres 701 and 702 are bonded together with their edges in closeproximity to each other with one continuous, or several (e.g., three)rigid, non-conductive spacer(s) affixed to the edges of each ofhemispheres 701 and 702 to maintain the spaced-apart relationship andthe electrical isolation. Alternatively a non-conductive adhesive can beused between the edges of hemispheres 701 and 702. Then vinyl skins 601and 602 with the geographical information are mounted over the twohemispheres as discussed above with respect to FIG. 11.

In this embodiment each hemisphere has three contact points affixed tothe inner edge of each, with hemisphere 701 having contact points 710,711 and 712, and hemisphere 702 having contact points 740, 741 and 742.Here, each hemisphere is shown with a small hole through the polar capto permit three insulated conductor cables 730 and 750, respectively, topass through and have one end of each insulated conductor connect to thethree points on the inner edge of the corresponding hemisphere. Theother end of each of cables 730 and 750 in-turn are connected to aseparate pair of switches in signal generator 722. The upper hemisphere701 has cable 730 connected to switches 770 and 771, while the lowerhemisphere 702 has cable 750 connected to switches 772 and 773.

By comparing FIG. 12 with FIG. 4 it can be seen that while theembodiment of FIG. 4 is for a single surface and FIG. 12 is for a pairof surfaces, the only wiring change between the signal generator of eachembodiment is the addition of a second pair of switches for the secondsurface for the embodiment of FIG. 12. The remainder of the signalgenerator in each instance is the same with amplifier 134 connected toboth pair of switches 770 and 771, and 772 and 773. This is possiblesince there is only one stylus 116 and only one point on the globe canbe selected at one time (i.e., the selected point can only be on onehemisphere at a time). Thus, each hemisphere is treated as anindependent location detection surface.

To make a determination as to which of hemispheres 701 and 702 the userhas pointed stylus 116, microprocessor 142 is programmed to make aseries of measurements. First, as in many of the embodiments discussedabove, with stylus 116 pointing at the selected point on one of thehemispheres, Signal_(FULL) and Signal_(OFFSET) are measured for eachhemisphere independently, and then the difference between those measuredvalues for each hemisphere (i.e., Signal_(FULL-701)-Signal_(OFFSET-701), and Signal_(FULL-702) -Signal_(OFFSET-702)) isdetermined and stored in RAM 144. In short, Signal_(FULL) is measured byapplying the 60 KHz AC signal to all of the contact points on thesurface, and Signal_(OFFSET) is measured will all of the correspondingswitch contacts in signal generator 722 for that surface open. Oncethose difference values are determined, each of those difference valuesis compared to a pre-selected threshold value. The threshold value isdetermined empirically and typically are the value measured when thestylus tip is within 0.10 inches from the surface. It is then notedwhich, if any, of those difference values exceeds the threshold and doesso with the greatest margin with the corresponding hemisphere beingidentified as the one to which stylus 116 is being pointed.

Once the hemisphere of interest has been determined, microprocessor 142then calculates the position selected by the sequence of calculationsoutlined above with respect to FIG. 4. Thus, four measurements,Signal_(FULL), signal_(OFFSET), Signal_(RAW-AB) and Signal_(RAW-AC) aremade on the identified hemisphere and the values of Signal_(NORM-AB) andSignal_(NORM-AC) are calculated as in equation 4 with those valuesdefining a unique location on that hemisphere.

The unique location provided by the values of Signal_(NORM-AB) andSignal_(NORM-AC), together with the results of the threshold test todetermine which hemisphere is of interest to the user, may then bemapped into a location on the globe by means of a look-up table for theselected hemisphere, if necessary, to obtain the longitude and latitudeof the point selected, in a standard globe coordinate system. Then, asdiscussed with respect to FIG. 11, microprocessor 142 can present theuser with information relative to the selected point from memory viaaudio video card 150 and speaker 154, or by any other desired media(e.g., printer, monitor, etc.) or combinations of media.

In addition to the user acting as an antenna and picking up atmosphericnoise and signals as described in the Background of the Invention above,there is another secondary effect that can potentially occur if the useris not grounded with respect to the system of the present invention.Since in the present invention the surface to which the user points theprobe, in the AC mode, is radiating a different signal at differentsurface coordinates, a portion of the user's hand, perhaps a finger orthumb, while holding the probe at the desired location may pick-up adifferent signal from another location spaced away from the location ofinterest. In such a situation the antenna of the probe can potentiallybe influenced by that secondary signal capacitively coupled from thesurface to the user and then coupled to the antenna of the probe. Thatsecondary signal could result in a modified signal being received by thesignal measurement stage 120. That modified signal from the surfacemight then be processed to identify a location other than the actuallocation to which the user has pointed the probe tip.

For example, assume that the user has pointed the probe tip at Chicagoon the surface of a globe of the present invention. In holding the probetip at that location the user's thumb might extend east and be close toDetroit while several of the user's fingers extend west of Chicagotoward Quincy, Ill. on the Mississippi River. What indeed might happenis that a mix of signals from the location to which the probe ispointed, together with a signal from each finger and the thumb of theuser could be received by the signal measurement stage 120 as anaveraged signal resulting in the identification of the selected point asa location between Detroit and Quincy, or even somewhere else on thesurface that is not even close to the location selected by the user,perhaps Tokyo. Even worse, the signal received by the antenna of theprobe may be so complex as a result of all of the various signalscoupled to it that the signal measurement stage is unable to identifyany location that corresponds to the combined signal. By inclusion ofthe mechanism to ground the user with respect to the system, asdiscussed below, this potential problem, as well as any influencecreated by atmospheric noise as discussed in the Background of theInvention will be resolved by virtually eliminating the other signalscoupled to the antenna of the probe from the user.

In each of the embodiments wherein a radiated AC signal is detected bystylus 116 acting as an antenna (see FIGS. 4, 5, 6, 7, 11 and 12),stylus 116 is coupled to demodulator 144 with a shielded cable 118.Shielded cable 118 has been included in an effort to prevent the lengthof cable 118 from acting as an antenna, in addition to stylus 116, andpicking-up signals some distance from and not emanating from thecorresponding surface of interest (i.e., 100, 400, 603, 701 or 702).

In prior art situations that require an antenna at the distal end of acable to use as a pointer in a system for locating the point to whichthe stylus is pointed, the internal circuit configuration of that stylusis very complex. FIG. 13 is a schematic representation of such a stylus916 used with the SEGA PICO interactive story book toy. Note that evenin an industry, the toy industry in this example, where it is imperativeto keep costs low to not price a product out of an intended marketplace,a relatively complex circuit has been used. The only saving grace,expense wise, is that the product was probably assembled by low paidworkings in a third world country.

There are several differences that can be seen between this design ofstylus 916 and stylus 116 of the present invention. First, and foremostis the active circuit design of the prior art that includes twotransistors, and specialty design IC, numerous capacitors, inductors andresistors, a power switch and a potentiometer requiring extensiveassembly, as opposed to the passive circuit design of the presentinvention. In addition to the active circuit design there is thenecessity of a formed metal shield 920 at the antenna end of stylus 916to exclude spurious responses from interfering with the signal receivedfrom the antenna. There is also a labor-intensive step of calibratingstylus 916 to the system with which it is to be used by means ofpotentiometer 922. Another added cost item is the use of a four wirecable 918 that is necessary to perform several functions: a shield; aline to carry the received signal back to the main chassis of theproduct; and two wires to carry power to stylus 920. Finally there isthe power switch 912 that needs to be depressed during use to powerstylus 916 which can present a problem if the intended user is a child,as is the case with the SEGA product.

FIG. 14a illustrates one embodiment of the combination of stylus 11 andshielded cable 118. In this view the distal end of stylus 116 is shownin dotted outline to illustrate the end of cable 118 in the interior ofthe distal end of stylus 116. In this embodiment shielded cable 118continues to near the extreme distal end of stylus 116 with the shieldintact and then a selected length of center conductor 802' is exposed toact as the antenna. At the proximate end of shielded cable 118, shield800 is grounded in signal measurement stage 120 and center conductor 802is connected to demodulator 144 to provide the input signal thereto.Thus, in this embodiment an signal that impinges along the length ofshielded cable 118 will not contribute to the signal detected by theantenna length of center conductor 802'. However, if the person holdingstylus 116 is inadvertently also acting as antenna and radiates some ofthe received signal to center conductor 802', that signal adds to thedesired signal from the surface of interest (e.g., surface 100). Then,depending on many factors including the ability of demodulator 144 toreject unwanted signal frequencies and noise, the position of stylus 116that is ultimately determined by the position location system of thepresent invention may not be as accurate as desired.

A first embodiment of this aspect of the present invention isillustrated in FIG. 14b. In this view the connections at the proximateend of shielded cable 118 are the same as in FIG. 14a. At the distal endof stylus 116 there are some changes that have been made to effect thegrounding of the user when holding stylus 116 to eliminate the parallelantenna effect inadvertently created by the user holding stylus 116 nearcenter conductor/antenna 802'. Here it can be seen that the distal endof shielded cable 118, in addition to having center conductor 802'exposed, has a portion of shield 800' exposed. In addition, stylus 116defines a hole 804 therethrough so that when a user holds stylus 116 aportion of one of the user's fingers must extend through hole 804 andmake contact with shield 800', thus grounding the user.

A second embodiment of this aspect of the present invention isillustrated in FIGS. 14c and 14d with FIG. 14d showing a cut-away viewof the distal end of stylus 116 to illustrate the internal configurationof this embodiment. In these views the connections at the proximate endof shielded cable 118 are the same as in FIGS. 14a and 14b. In FIG. 14cstylus 116 includes three portions: tip 810; main body 812; andconductive grip 806 that extends around stylus 116 at the point of theuser's grasp. In FIG. 14d a portion of tip 810 and conductive grip 806have been cutaway to illustrate the internal structure of the distal endof stylus 116. The internal arrangement is similar to that of FIG. 14bwith the exception of the length of shield 800' that has been exposedand dressing of a pig-tail 808 of shield 800' back beneath conductivegrip 806. Thus, when the user grasps stylus 116 with conductive grip 806the user is grounded by the electrical interaction of conductive grip806 and shield 800' and pig-tail 808. Various structures and materialscould be used to conductive grip 806 varying from spring loaded metalrings to conductive polymers. One such conductive polymer might be acarbon impregnated Kraton D-2104 polymer (e.g., RTP 2799X66439).

Additionally, it is well known by those skilled in the art how one wouldstore data relative to points on any surface that might be employed withthe present invention, as would be look-up tables to convert onecoordinate system for a surface to another coordinate system.

While the discussions of the various embodiments of the presentinvention presented above address a variety of shapes and applicationsfor the present invention, the shapes and applications addressed areclearly not an exhaustive list. One could easily extend such lists tomany other shapes and applications and the techniques discussed abovecould easily be extended to each of them. Thus, the present invention isnot limited solely to the scope of what has been discussed above, butrather is only limited by the scope of the claims appended hereto.

What is claimed is:
 1. An electrographic sensor unit for use indetermining the position of a selected point, which comprises:a layer ofa conductive material having an electrical resistivity and a surface; Kspaced apart contact points electrically interconnected with said layerof conductive material; a processor connected to said K spaced apartcontacts and disposed to selectively apply a signal to N of said Kcontact points relative to a signal neutral point, and where N has aninteger value of 3 to K; and a probe assembly including:a cable having afirst conductor and a second conductor with the proximate end of saidone conductor coupled to said processor and the proximate end of saidsecond conductor connected to said signal neutral point; and a styluscoupled to said cable and incorporating therein distal ends of saidfirst and second conductors with the distal end of said first conductordisposed to receive signals from said layer when said contact pointshave signals selectively applied thereto and said user positions saidstylus in vicinity of a user selected point on said surface, and withthe distal end of said second conductor disposed to be contacted by saiduser when holding said stylus to connect said user to said signalneutral point; wherein said position of said stylus relative to saidsurface of said layer is determinable by said processor from signalsreceived from said first conductor of said stylus each in relation to asimilar excitation of J different pairs of said K contact points undercontrol of said processor, where J is an integer between 2 and (N-1). 2.An electrographic sensor unit as in claim 1 wherein:said processorselectively applies AC signals to selected ones of said K spaced apartcontact points; said distal end of said first conductor detects signalsradiated from said layer of conductive material as an antenna withoutmaking physical contact with said layer; and said distal end of saidsecond conductor when contacted by said user connects said user to saidsignal neutral point to minimize any noise radiated by said user frombeing received by said distal end of said first conductor and beingdelivered to said processor.
 3. An electrographic sensor unit as inclaim 1 wherein said stylus further includes an electrically conductivecontact making electrical contact to said distal end of said secondconductor, and located externally and positioned to be contacted by theuser during use of said stylus.
 4. An electrographic sensor unit as inclaim 3 wherein said electrically conductive contact is a flexibleconductive polymer that encircles said stylus at a position to maximizethe user's comfort when holding said stylus.
 5. An electrographic sensorunit for use in determining the position of a selected point, whichcomprises:a layer of a conductive material having an electricalresistivity and a surface; three spaced apart contact pointselectrically interconnected with said layer of conductive material; aprocessor connected to said three spaced apart contacts and disposed toselectively apply a signal to each of said three contact points relativeto a signal neutral point; and a probe assembly including:a cable havinga first conductor and a second conductor with the proximate end of saidone conductor coupled to said processor and the proximate end of saidsecond conductor connected to said signal neutral point; and a styluscoupled to said cable and incorporating therein distal ends of saidfirst and second conductors with the distal end of said first conductordisposed to receive signals from said layer when said contact pointshave signals selectively applied thereto and said user positions saidstylus in vicinity of a user selected point on said surface, and withthe distal end of said second conductor disposed to be contacted by saiduser when holding said stylus to connect said user to said signalneutral point; wherein said position of said stylus relative to saidsurface of said layer is determinable by said processor from signalsreceived from said first conductor of said stylus each in relation to asimilar excitation of two different pairs of said three contact pointsunder control of said processor.
 6. An electrographic sensor unit as inclaim 5 wherein:said processor selectively applies AC signals toselected ones of said three spaced apart contact points; said distal endof said first conductor detects signals radiated from said layer ofconductive material as an antenna without making physical contact withsaid layer; and said distal end of said second conductor when contactedby said user connects said user to said signal neutral point to minimizeany noise radiated by said user from being received by said distal endof said first conductor and being delivered to said processor.
 7. Anelectrographic sensor unit as in claim 5 wherein said stylus furtherincludes an electrically conductive contact making electrical contact tosaid distal end of said second conductor, and located externally andpositioned to be contacted by the user during use of said stylus.
 8. Anelectrographic sensor unit as in claim 7 wherein said electricallyconductive contact is a flexible conductive polymer that encircles saidstylus at a position to maximize the user's comfort when holding saidstylus.
 9. An electrographic sensor unit in the form of a globe for usein determining the position of a user selected point on the surfacethereof, which comprises:a sphere formed of a layer of a conductivematerial having a substantially uniform electrical resistivity and anouter surface; a set of four spaced apart contact points electricallyinterconnected with said layer of conductive material of said sphere; aprocessor connected to said set of four spaced apart contacts anddisposed to selectively apply a signal to each of said four contactpoints relative to a signal neutral point; and a probe assemblyincluding:a cable having a first conductor and a second conductor withthe proximate end of said one conductor coupled to said processor andthe proximate end of said second conductor connected to said signalneutral point; and a stylus coupled to said cable and incorporatingtherein distal ends of said first and second conductors with the distalend of said first conductor disposed to receive signals from said layerwhen said contact points have signals selectively applied thereto andsaid user positions said stylus in vicinity of a user selected point onsaid sphere, and with the distal end of said second conductor disposedto be contacted by said user when holding said stylus to connect saiduser to said signal neutral point; wherein said position of said stylusrelative to said surface of said sphere is determinable from threesignals received from said stylus by said processor each in relation toa similar excitation of three different pairs of said four contacts onsaid sphere by said processor.
 10. An electrographic sensor unit as inclaim 9 wherein:said processor selectively applies AC signals toselected ones of said four spaced apart contact points; said distal endof said first conductor detects signals radiated from said layer ofconductive material as an antenna without making physical contact withsaid layer of said sphere; and said distal end of said second conductorwhen contacted by said user connects said user to said signal neutralpoint to minimize any noise radiated by said user from being received bysaid distal end of said first conductor and being delivered to saidprocessor.
 11. An electrographic sensor unit as in claim 9 wherein saidstylus further includes an electrically conductive contact makingelectrical contact to said distal end of said second conductor, andlocated externally and positioned to be contacted by the user during useof said stylus.
 12. An electrographic sensor unit as in claim 11 whereinsaid electrically conductive contact is a flexible conductive polymerthat encircles said stylus at a position to maximize the user's comfortwhen holding said stylus.
 13. An electrographic sensor unit for use indetermining the position of a selected point, which comprises:a firstlayer of a conductive material having an electrical resistivity and afirst surface; a first set of three spaced apart contact pointselectrically interconnected with said first layer of conductivematerial; a second layer of a conductive material having an electricalresistivity and a second surface; a second set of three spaced apartcontact points electrically interconnected with said second layer ofconductive material; a processor connected to each of said first andsecond sets of three spaced apart contacts and disposed to selectivelyapply a signal to each of said three contact points in each of saidfirst and second sets thereof; and a probe assembly including:a cablehaving a first conductor and a second conductor with the proximate endof said one conductor coupled to said processor and the proximate end ofsaid second conductor connected to said signal neutral point; and astylus coupled to said cable and incorporating therein distal ends ofsaid first and second conductors with the distal end of said firstconductor disposed to receive signals from said layer with said usserselected point when said corresponding set of contact points havesignals selectively applied thereto and said user positions said stylusin vicinity of a user selected point on one of said first and secondsurfaces, and with the distal end of said second conductor disposed tobe contacted by said user when holding said stylus to connect said userto said signal neutral point; wherein identification of which of saidfirst and second surfaces said stylus is adjacent to is accomplished bysaid processor by independently measuring two signals from each of saidfirst and second layers received by said stylus, combining said signalsfrom the same layer independent of the signals received from the otherlayer to form a first and a second comparative value with each saidcomparative value associated with a different one of said first andsecond layers, and independently comparing each of said first and secondcomparative values to a preselected threshold value with the layerassociated with the one of said first and second comparison value thatis greatest and is greater than said threshold being the layer saidstylus is closest to and therefore an identified layer of said first andsecond layers; and wherein said position of said stylus relative to saididentified one of said first or second layers is determinable by saidprocessor from signals received from said stylus each in relation to asimilar excitation of all of said three contact points on the identifiedone of said first and second layers and two different pairs of saidthree contact points on the identified one of said first and secondlayers under control of said processor.
 14. An electrographic sensorunit as in claim 13 wherein:said processor selectively applies ACsignals to selected ones of said four spaced apart contact points; saiddistal end of said first conductor detects signals radiated from saidlayer of conductive material as an antenna without making physicalcontact with said layer of said sphere; and said distal end of saidsecond conductor when contacted by said user connects said user to saidsignal neutral point to minimize any noise radiated by said user frombeing received by said distal end of said first conductor and beingdelivered to said processor.
 15. An electrographic sensor unit as inclaim 13 wherein said stylus further includes an electrically conductivecontact making electrical contact to said distal end of said secondconductor, and located externally and positioned to be contacted by theuser during use of said stylus.
 16. An electrographic sensor unit as inclaim 15 wherein said electrically conductive contact is a flexibleconductive polymer that encircles said stylus at a position to maximizethe user's comfort when holding said stylus.